WIRELESS TELEPHONY — SLAUGHTER. 
185 
While this voltage regulating device operates in an extremely 
satisfactory manner, the excessive speed variation of the generator 
imposes certain other requirements on the generator design which 
make it desirable to reduce this speed variation. The development 
of a constant-speed air fan has progressed practically to the point 
where designs are available for an air fan which will give an almost 
exactly constant rotational speed with a range of air speeds of at 
least 4 to 1. The use of such an air fan on future wind-driven 
generators for airplanes is a practical certainty. 
Various alternatives, such as storage batteries and a dynamotor, 
or a double voltage generator driven directly from the airplane 
engine, were considered and rejected in favor of the generator above 
described. 
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Fi<j. la. — Voltage characteristic, wind-driven generator. 
TRANSMITTING SET. 
The circuits of the transmitting set are shown schematically in 
figure 3. A single vacuum tube of a special type is used for fur- 
nishing the radio frequency current and a similar tube is used for 
modulating the radio frequency current. The method of modula- 
tion employed is known as the " constant current " method and may 
be described briefly as follows : 
The plate circuits of the vacuum tube oscillator and modulator 
are connected in parallel and are supplied through an inductance 
coil, which tends to maintain the total current constant. The plate 
current taken by the modulator tube is controlled by its grid voltage, 
which in turn is determined by the operation of the telephone trans- 
mitter. These voice frequency fluctuations of the modulator plate 
current cause corresponding variations in the direct current supplied 
to the plate circuit of the oscillator tube, the sum of the two currents 
